1. Field of the Invention
This invention relates to devices for driving a photographic optical system of the camera to move stepwise by utilizing the torque obtained with an energized coil in a magnetic field so as to effect focusing (hereinafter called "electromagnetically driven lens barrel"), and more particularly, to an arrangement of a switch for setting an initial position in that kind of lens barrels.
2. Description of the Prior Art
It is known, in U.S. Pat. No. 4,560,266 issued on Dec. 24, 1985 and assigned to the assignee of the present invention, to provide this kind of electromagnetically driven lens barrel in which the barrel is first moved forward stepwise to an in-focus position, and after an exposure has been made, it is then reset in an initial position, thus being readied for the next shot. To detect when the resetting of the barrel has been completed, it has been customary to arrange an actuator for a switch on the barrel at a radial side thereof so that the movable contact of the switch is depressed by the actuator when the barrel reaches the initial position.
In the conventional electromagnetically driven lens barrel, as shown in FIG. 4A, the barrel 102 containing the lens 101 has a cam follower pin 102a pressed against the camming surface 103a of the cam ring 103 by a return spring (not shown). By turning the cam ring 103 in a direction indicated by arrow "i", the barrel 102 is axially moved forward to effect focusing. After an exposure with the lens 101 in focus, has been completed, the cam ring 103 is turned again in the same direction of arrow "i", until the camming surface 103a moves away from the pin 102a. Thus, as the pin 102a drops from the crest 103b to the valley 103c, the barrel 102 is reset in the initial position. By the way, for the purposes of minimizing the frictional force of the pin 102a and of enabling the height of the pin 102a to be adjusted by turning it about its own axis, the end of the pin 102a is formed to a semi-spheric shape. From this reason, it results that during the resetting operation, soon after the vertex of the lobe 102a overruns the terminal edge 103b of the camming surface 103a, an impulse of force F2 perpendicular to the force F1 of pressing the barrel 102 against the cam ring 103 is given to the cam ring 103 as shown in FIG. 4B, causing the latter to go beyond the initial position despite no electromagnetic force being applied thereto. It has, therefore, been impossible to insure that the barrel 102 is reset in the initial position with high accuracy and reliability.